Drive motors of an auxiliary assembly of a motor vehicle, in particular BLDC motors or EC motors, have previously been described. Such drive motors generally comprise a motor stator with at least one stator coil and a motor rotor that is provided with at least one permanent magnet or which is itself permanent-magnetic, wherein the stator coil is configured to be energized with a predefined coil voltage and a predefined constant start-up rotation frequency by a control electronics system and a power electronics system to generate a rotating exciter field to drive the motor rotor. By energizing the stator coil or a plurality of stator coils in a certain pattern, it is possible to generate a rotating field, for example, by a multi-phase and/or a multi-strand coil system set configured as a star or a delta connection, by which field the permanently magnetic motor rotor can be “taken along” or driven. This directed energizing of the coils or the commutation of the coil voltage is effected through a directed switching of power semiconductors.
The term coil voltage as used herein refers to a voltage pulse or to an effective voltage generated by a clocking of a DC supply voltage according to a duty cycle, for example, at a frequency of several kHz. The duty cycle indicates the ratio of an activation time or of drive pulses to a period duration of the cyclic control and thereby determines the effective voltage that can be applied to the stator coil in this manner. The clocking is performed using a control electronics system and/or a power electronics system. The control electronics system can thus drive the power electronics system with a defined duty cycle to control the drive motor with a variable power corresponding to a pulse width modulation. A control starts, for example, with a duty cycle of 22/250 which corresponds to about 10% of the supply voltage. A DC supply voltage of 12 volt can thereby be clocked to a much lower effective voltage applied to the coil, namely 1.2 volt. A maximum coil voltage can be reached by a non-clocked transmission of the DC supply voltage to the stator coil by the power electronics system.
It is necessary for the rotating field to be matched with the position or movement of the motor rotor to start-up or start the drive motor. The rotor position may be determined either by a sensor or in a sensor-less manner therefor, for example, by a voltage reinduced in the stator coils. Due to the very small reinduced signals, however, such a measurement generally fails at a standstill or at very low rotational speeds of the motor rotor which is the case when starting up the drive motor of an auxiliary assembly of a motor vehicle. The determination of the rotor position therefore generally requires a minimum rotational speed of about 10% of the maximum rotational speed of the rotor.
In known methods, to start-up such a drive motor, at least up to a minimum rotational speed, the stator coils are therefore energized with a defined, generally very high start-up voltage value to guarantee a start-up of the motor rotor under all circumstances. In particular with regard to boundary conditions which may influence a start-up of the rotor such as temperature, load, or friction, a coil voltage is predefined for the start process that is so high that a magnetic force field of such a magnitude is generated that the motor rotor would only fail to start if a mechanical blocking of the motor rotor existed. The motor rotor would start in all other cases.
After the start-up of the motor rotor, the stator coils are controlled in an unregulated manner or “blindly” according to a rigid pattern scheme or at a defined rotation frequency until the motor rotor has reached a minimum rotational speed and the signal reinduced in the coils is measurable to determine the rotor position so that a sensorless detection of the rotor position is possible. It is then possible to switch to the regulated commutated operation.
A distinction must therefore be made between a first start phase, in which the motor rotor is caused to rotate from a standstill, and a second start phase, in which the motor rotor has reached a minimum rotational speed and the drive motor can thus transition to a regulated mode. The present invention exclusively relates to the above-mentioned first start phase.
A method for starting a DC motor is described in DE 199 36 755 A1 in which, for the start-up of the motor rotor, the stator coils are driven “blindly” according to a rigid pattern, wherein the coils are energized with such a high coil voltage that the rotor, provided the rotor is not blocked, would start in any case. The rotation of the rotor is then detected in a sensorless manner using a voltage reinduced in the coils so that a regulation of the energizing of the coils can be performed via a microcontroller. The motor is deactivated if a start-up of the rotor is detected.
Such a highly over-excited control of the coils for a start of the rotor leads to a high power consumption due to the great proportion of reactive power, so that the efficiency of the drive motor of an auxiliary assembly of a motor vehicle is ultimately reduced. An over-excited activation leads to an increased power loss or loss in efficiency in the case of a low load being applied or of a relatively free-running motor rotor that would already start at a very low coil voltage.